This invention relates to systems for automatically controlling the operation of a plurality of motors and, more particularly, to systems for automatically controlling the starting and stopping of a plurality of electric motors.
The main function of a system for automatically controlling the operation of a plurality of motors is to systematically start the motors in accord with the operating demand therefore and to systematically stop the motors as the operating demand diminishes. The useful life of an electric motor diminishes as its operating time accumulates and the life is rapidly shortened by frequent starts with insufficient time between prior stops to permit the cooling of the motor parts. As such, the order that is followed in the starting and stopping of the motors in a control system is important in prolonging the life of each motor.
The ideal system for controlling a plurality of motors would be one which, over a prolonged period of time, subjects all of the controlled motors to an equal number of starts and to an equal amount of operating time, and all while maximizing the time period between successive stops and starts for each motor being controlled. This would serve to prolong the useful life of each motor and would greatly facilitate the establishment and the performance of inspection, repair and maintenance schedules for both the controlled motors and the equipment operated thereby.
Most systems for automatically controlling the operation of a plurality of motors use a motor starting sequence which is based on a predetermined "cyclic" or "repeating" order for systematically and sequentially starting all of the motors being controlled by the system. The starting sequence is changed in such systems in accord with a control cycle that contemplates some recurring event, and this is usually when the first motor rendered operational in accord with the starting sequence is thereafter deenergized.
For example, the five motors required at a peak operating demand on a conventional system for automatically controlling such motors may be identified as motors Nos. 1 through 5 inclusive, and in the order in which they are sequentially energized to fulfill a peak demand when the system is initially rendered operational. The "cyclic" or "repeating" motor order on which the starting sequence is based will then be the repeating sequence of motors Nos. 1 through 5 inclusive, or motor #1, #2, #3, #4, #5, #1, #2, #3, #4, #5, #1, #2, #3, . . . etc. The normal pattern followed in changing the motor starting sequence in accord with the control cycle for such conventional systems is to provide an advance of one motor in the repeating order with each successive change in the starting sequence. As such, in the conventional system contemplated, the initial starting sequence for a peak demand situation would be motor #1, #2, #3, #4 and #5 in that order, and the next or second starting sequence would be motor #2, #3, #4, #5 and #1 in that order. With succeeding control cycles, the starting sequence would accordingly change by advancing one motor number in the repeating order with each successive cycle.
The conventional automatic control systems use a motor stopping sequence which is based on an order that is the reverse of that followed in starting the motors. This simply means that between the changes in the motor starting sequence, and where one or more of the controlled motors has theretofore been energized in accord with the motor starting sequence then governing the operation of the motors, the motor which is stopped in response to a diminishing motor operating demand will be the last motor that was energized in accord with the motor starting sequence then governing the system. Simultaneously, any increased operating demand before the first motor started in accord with the governing starting sequence is deenergized, will be in accord with the starting sequence which is then governing the system.
The conventional control systems are less than satisfactory and this is particularly so in situations where several motors are being controlled and the motor operating demand on the system fluctuates over long periods of time and rarely diminishes below a demand for at least one motor. Under such circumstances, one or more of the controlled motors will be subjected to a far greater amount of wear than others because they are subjected to more frequent starts and stops. For example, in the conventional system for controlling the five motors referred to heretofore, the starting sequence governing the system during the initial control cycle contemplated the systematic and sequential starting of motors #1, #2, #3, #4 and #5 in that order in order to fulfill a peak demand on the system. If the maximum demand imposed on the system when the initial starting sequence was governing the system was only for the operation of three motors, and if the demand fluctuated several times from one to three motors before the starting sequence was changed by the deenergizing of motor #1, then motors #2 and #3 would have been subjected to several starts and stops and the #1 motor would have remained operational throughout the initial cycle. Simultaneously, motors #4 and #5 would have remained inoperative. In many situations the demand for the operation of at least one motor may continue for several months. This, of course, would preclude an opportunity for the starting sequence to be changed if the change is keyed to a deenergizing of the first motor started in the sequence and it would also subject at least some of the other motors to frequent starts and stops in accord with the fluctuating demand and thus to the accelerated wear which arises from such occurrences.